There are over four-hundred nuclear power plants worldwide, providing nearly twenty percent of the world's electricity. Nuclear power plants function much like power-generating plants that are fueled by coal or oil. That is, either type of power plant generates heat. The heat is used to heat water and produce steam, or to heat a gas. The steam or the gas, as the case may be, drives one or more turbines which in turn generate electricity. The difference, of course, is that heat is generated at a nuclear power plant by nuclear reactions (i.e., induced fission) instead of by burning coal or oil.
Induced fission takes place in the reactor. The fuel for the reactor is provided by a suitable radioactive material (e.g., uranium-235 or plutonium-239) typically formed into either rods or “pebbles” that are arranged within the core of the reactor. As the fuel fissions, neutrons are released which bombard the nuclei of the other fuel atoms in the core of the reactor. The bombarded nuclei absorb the neutrons causing the nuclei to become unstable and split, releasing one or more neutrons which bombard the nuclei of yet other fuel atoms, and so on. The split atoms release energy in the form of radiation and heat.
During operation of the reactor, a coolant is passed through the core of the reactor to maintain the reactor at a normal operating temperature and keep it from overheating. The coolant may be either a gas-phase coolant (e.g., helium) or a liquid-phase coolant (e.g., water) that flows into the reactor, absorbs the heat produced during induced fission, and flows out of the reactor.
The heated coolant that flows out of the reactor may then be passed through a heat-exchanger. Water is also provided to the heat exchanger to absorb heat from the heated coolant. The coolant is then recirculated into the reactor. The heat absorbed by the water produces steam. This steam is used to drive the turbines that operate the generator and generate electricity. Alternatively, in a direct cycle gas-cooled reactor the cooling fluid is used directly to drive the turbines.
In some circumstances, the flow of coolant into the reactor may be insufficient to cool the reactor. As an example, the flow of coolant into the reactor may be interrupted by a blockage in the pipe system or failure of a pump, reducing or altogether stopping the flow of coolant into the reactor. When this happens, the reactor must be shut down so that the reactor does not overheat.
The reactor is provided with one or more control elements that can be lowered into the reactor to slow and eventually stop the reactions occurring therein when the reactor exceeds a safe operating temperature. Control elements may be made from a variety of materials that absorb free neutrons. When the control elements are lowered into the reactor, the control elements absorb the neutrons instead of the neutrons being absorbed by the fuel, causing the reactor to shut down.
Typically, a number of monitors are used to determine how much heat is being generated in the reactor. For example, the monitors may measure the temperature in the reactor. When the temperature in the reactor exceeds safe operating conditions, the monitors signal an emergency response system which in turn lowers the control elements into the reactor to shut it down. For safety reasons redundant monitors are commonly provided so that if one fails, another of the monitors will still signal the emergency response system of the unsafe operating condition so that it can shut down the reactor. However, the monitors must still signal the emergency response system when the unsafe condition occurs, thereby introducing delay and another potential point of failure. In addition, such redundant monitors can be complex and therefore expensive.